1. Field of the Invention
This invention relates to a semiconductor photodetector for converting light into electricity at high speed, which is suitable for use in optical communications, and more particularly, to a semiconductor photodetector which is capable of selective photoelectric conversion of light on the longer wavelength side in two-wavelength multiplex optical communications, thereby extracting the signal on this longer wavelength side, and to a method for manufacturing same and a photodetector module containing the photodetector.
2. Description of Related Art
One example of a conventional semiconductor photodetector (hereinafter, referred to as xe2x80x9cphotodetectorxe2x80x9d or xe2x80x9cphoto detecting elementxe2x80x9d) is described in the reference source (Hiker tsushin soshi kogaku, xe2x80x9cHakko/Juko soshixe2x80x9d, pp. 371-372, pp. 384, Kogaku Toshokan). FIG. 6 is a general sectional view showing the composition of-the photodetector according to this source. This photodetector 100 is a planar surface-exposure-type photodetector using an InGaAs-type compound semiconductor. This photo detecting element 100 comprises an n-InP buffer layer 103, an n-InGaAs light-absorbing layer 105, an n-InP window layer 107, and an insulating film 109, laminated in this order onto an n+ InP substrate 101. A portion of the n-InP window layer 107 is constituted by a p+ diffused region 111. Furthermore, a p-side electrode 113 which is electrically connected to the p+ diffused region 111 is provided on the p+ diffused region 111, and an n-side electrode 115 is provided on the rear face of the n+-InP substrate 101. A short-wavelength shielding filter 117 formed by a dielectric film is provided on the light receiving surface of the photo detecting element 100, on top of the insulating film 109. The p-side electrode 113 is exposed outside the short-wavelength shielding filter 117 (FIG. 6).
This photodetector 100 uses the photovoltaic effect of a pn junction diode. In two-waveleilgth multiplex optical communications, it performs photoelectric conversion of light on the longer wavelength side, selectively. Light on the shorter wavelength side is shut out by being reflected at the exposed surface of the aforementioned short-wavelength shielding filter 117, so only light on the longer wavelength side reaches the light-absorbing layer 105. Therefore, in the light-absorbing layer 105, only this light on the longer wavelength side is photoelectrically converted, and hence it is possible to extract the electrical signal on the longer wavelength side alone.
Moreover, FIG. 7 is a general compositional view of a conventional semiconductor photodetector module (hereinafter, referred to as xe2x80x9cmodulexe2x80x9d) provided with a short-wavelength shielding filter, and it shows a sectional view thereof. This module 200 comprises: an optical fiber 201 having a light emitting end face 201a; a ferrule 203 for supporting the optical fiber 201 in the center portion thereof; a lens 205 for focusing light emitted from the optical fiber 201; a semiconductor photodetector 207 onto which this focused light is irradiated; and a header 209 onto which the photodetector 207 is fixed. In this module 200, a short-wavelength shielding filter 211 is provided between the lens 205 and the semiconductor photodetector 207 (FIG. 7).
In this module 200, if multiplexed light comprising two light beams of different wavelengths is emitted from the optical fiber 201, then the light on the shorter wavelength side in the light focused by the lens 205 will be shut out by the short-wavelength shielding filter 211. Therefore, light on the longer wavelength side will be irradiated onto the photodetector 207. In this way, only light on the longer wavelength side is photoelectrically converted and so only the signal corresponding to this light can be extracted.
However, in a conventional photodetector as described above, a short-wavelength shielding filter made from a dielectric film is provided in order to shut out light on the shorter wavelength side from the light receiving surface of the element. The material used as the short-wavelength shielding filter for a semiconductor photodetector of this composition is a dielectric film. This dielectric film is formed from a different material to the dielectric films used in standard wafer processing, such as silicon oxide films, silicon nitride films, or the like. If a dielectric film forming a filter is provided on an underlying layer, then the difference in coefficient of expansivity between the filter and the underlying layer on which the filter is provided will be large, and therefore problems will arise in that the structural and operational reliability of the photodetector will deteriorate.
Moreover, processing problems also arise in that different etching methods are used for the filter and the other layers constituting the photodetector. Consequently, there is a possibility that the manufacturing equipment and processing involved will become more complex, thereby causing costs to rise also.
Furthermore, in a photodetector module having a structure wherein a short-wavelength shielding filter is provided between the semiconductor photodetector and the lens, increase in costs is unavoidable due to the need to provide a new, special filter in the conventional module structure.
There are also cases where light on the shorter wavelength side is reflected at the surface of the filter and is returned back to the transmitting side. This back-reflected light causes noise, and may degrade the reliability of the optical communications system.
Moreover, attenuation occurs in the light on the longer wavelength side transmitted by the filter, and therefore degradation of the signal may also occur.
This invention was devised with the foregoing in view. Therefore, it is an object of the invention to provide a semiconductor photodetector having a structure whereby light on the longer wavelength side can be photoelectrically converted and output reliably, whilst improving the reliability of the photodetector (or photo detecting element) in terms of its structure and operational performance, by means of a simple manufacturing process and inexpensive manufacturing costs.
It is a further object of this invention to provide a method for manufacturing a semiconductor photodetector (or photo detecting element) of this kind, inexpensively.
It is yet a further object of this invention to provide a photodetector (or photo detecting element) module for carrying out optical communications, whereby the reliability of an optical communications system is not degraded and no signal deterioration occurs.
According to a first aspect of the present invention, there is provided a semiconductor photodetector having the following composition. A first light-absorbing layer, a buffer layer of a second conductivity type, a second light-absorbing layer of a second conductivity type and a window layer of a second conductivity type are laminated successively onto a first principal surface of a substrate of a first conductivity type. The first light-absorbing layer comprises a region of a first conductivity type and a region of a second conductivity type provided in a laminated fashion in this order from the substrate. A diffused region of a first conductivity type having a depth extending from the upper face of the window layer to the interface between the window layer and the second light-absorbing layer is provided in a portion of the window layer. A main electrode of a first conductivity type, electrically connected to the diffused region, is provided on the diffused region; and a main electrode of a second conductivity type, electrically connected to the window region, is provided on the window region. In this semiconductor photodetector, the energy gap wavelength of the second light-absorbing layer is longer than the energy gap wavelength of the first light-absorbing layer, and the energy gap wavelength of the first light-absorbing layer is longer than the respective energy gap wavelengths of the substrate, buffer layer and window layer.
For example, multiplexed light comprising light of two different wavelengths is irradiated via the second principal surface of the substrate of this semiconductor photodetector. Here, the light on the shorter wavelength side has a shorter wavelength than the energy gap wavelength of the first light-absorbing layer and the light on the longer wavelength side has a wavelength that is longer than the energy gap wavelength of the first light-absorbing layer, but shorter than the energy gap wavelength of the second light-absorbing layer. The light having a wavelength shorter than the energy gap wavelength of the first light-absorbing layer is absorbed by the first light-absorbing layer. The light having a wavelength longer than the energy gap wavelength is transmitted. Therefore, the light on the shorter wavelength side in the incident light is absorbed by the first light-absorbing layer and the light on the longer wavelength side, which is transmitted by the first light-absorbing layer, is absorbed by the second light-absorbing layer. The light on the longer wavelength side absorbed by the second light-absorbing layer is extracted as an electrical signal from the main electrode of the first conductivity type and the main electrode of the second conductivity type, by means of photoelectric effect.
Therefore, it is possible to achieve a structure whereby light on the longer wavelength side can be photoelectrically converted and output reliably, without using a short-wavelength shielding filter composed of a dielectric film. Moreover, since the first light-absorbing layer for shutting out the light on the shorter wavelength side can be constituted by a material which does not increase the disparity in coefficient of expansion between the layers forming the photo detecting element, it is possible to increase the reliability of the photo detecting element in terms of its structure and its operating characteristics. Moreover, since no short-wavelength shielding filter is used in this photo detecting element, manufacture is simple and manufacturing costs are inexpensive.
Furthermore, carriers generated by the light on the shorter wavelength side absorbed by the first light-absorbing layer are trapped in the pn junction region formed by the region of the second conductivity type and the region of the first conductivity type. Therefore, the carriers do not travel towards the second light-absorbing layer. Consequently, it is possible to extract an electrical signal corresponding to the longer wavelength side which does not contain any electrical signal relating to the shorter wavelength side.
Preferably, the semiconductor photodetector (or photo detecting element) having the aforementioned structure, may also comprise: an auxiliary electrode of a first conductivity type provided on the second principal surface of the substrates and wiring or interconnection for providing a shorting connection between the auxiliary electrode of a first conductivity type and the main electrode of a second conductivity type.
The carriers generated by the light on the shorter wavelength side absorbed by the first light-absorbing layer are trapped in the pn junction region formed by the region of a first conductivity type and the region of a second conductivity type. The trapped carriers flow as a shorting current along the wiring or interconnection providing a shorting connection between the auxiliary electrode of a first conductivity type and the main electrode of a second conductivity type. Therefore, the carriers can be removed without accumulating in the pn junction region. Consequently, carriers originating in the light on the shorter wavelength side do not reach the vicinity of the second light-absorbing layer and the diffused region of the first conductivity type, where the light on the longer wavelength side is absorbed and converted to an electrical signal. Accordingly, it is possible to extract reliably, from the main electrode of a first conductivity type and the main electrode of a second conductivity type, a signal corresponding to the longer wavelength side only, which does not contain any signal relating to the shorter wavelength side.
According to a second aspect of this invention, there is provided a semiconductor photodetector (or photo detecting element) having the following composition. The photodetector comprises a first wave guide section and a second wave guide section provided on the first principal surface of an underlying layer, in a consecutive fashion in the direction of travel of incident light. In this photodetector, the first wave guide section is provided by laminating, at the least, a first light-absorbing layer of a first conductivity type, a first window layer of a second conductivity type and a first electrode of a second conductivity type, in this order. The second wave guide section is provided by laminating, at the least, a second light-absorbing layer of a first conductivity type, a second window layer of a second conductivity type and a second electrode of a second conductivity type, in this order. The underlying layer comprises a substrate of a first conductivity type, a buffer layer of a first conductivity type laminated onto the first principal surface of the substrate, and a third electrode of a first conductivity type provided on the second principal surface of the substrate. The energy gap wavelength of the second light-absorbing layer may be greater than the energy gap wavelength of the first light-absorbing layer, the energy gap wavelength of the first light-absorbing layer may be greater than the respective energy gap wavelengths of the substrate, buffer layer, first window layer and second window layer, and furthermore, the incident light may be entered from the end face of the first wave guide section which is opposite to the end face thereof adjacent to the second wave guide section.
By this means, if, for example, multiplexed light comprising light of two different wavelengths is used as the incident light, then the wavelength of the light on the shorter wavelength side is set such that it is shorter than the energy gap wavelength of the first light-absorbing layer, whilst the wavelength of the light on the longer wavelength side is set such that it is longer than the energy gap wavelength of the first light-absorbing layer, but shorter than the energy gap wavelength of the second light-absorbing layer.
Firstly, the incident light enters the first wave guide section, and light on the shorter wavelength side is absorbed by the first light-absorbing layer, whereas light on the longer wavelength side is transmitted. The light transmitted by the first wave guide section then enters into the second wave guide section via the end face thereof. The light on the longer wavelength side is absorbed by the second light absorbing layer. The light on the shorter wavelength side absorbed by the first light-absorbing layer can be extracted as a shorter wavelength side electrical signal by means of the first electrode of a first conductivity type in the first wave guide section and the third electrode of a second conductivity type provided on the second principal surface of the underlying layer. Furthermore, the light on the longer wavelength side absorbed by the second light-absorbing layer can be extracted as a longer wavelength side electrical signal by means of the second electrode of a first conductivity type in the second wave guide section and the third electrode. Therefore, when incident light enters the semiconductor photodetector (or photo detecting element), the respective lights can be converted photoelectrically in a selective manner, thereby enabling electrical signals to be extracted separately.
Moreover, since it is not necessary to use a filter, or the like, for shutting out light of the other wavelength in order to extract light of one wavelength in the form of an electrical signal, manufacture is simple and manufacturing costs are also inexpensive. Moreover, since each of the layers constituting the photodetector can be formed from materials which do not produce a large coefficient of expansivity with respect to mutually adjoining layers, it is possible to increase the structural and operational reliability of the photodetector.
Moreover, if, rather than two-wavelength light, multiplexed light containing more wavelengths is used as the incident light, it is possible to provide a number of wave guide sections on the first principal surface of the underlying layer corresponding to the number of wavelengths contained in the incident light. By this means, the lights contained in the incident light can each be photoelectrically converted, respectively, and extracted separately as electrical signals originating in the respective wavelengths.
According to a third aspect of this invention, a method for manufacturing the semiconductor photodetector (or photo detecting element) described above is provided. This manufacturing method comprises the steps of: successively laminating a preliminary layer for a first light-absorbing layer of a second conductivity type, a buffer layer of a second conductivity type, a second light-absorbing layer of a second conductivity type and a window layer of a second conductivity type, onto a first principal surface of a substrate of a first conductivity type; forming a diffused region having a depth extending from the upper face of the window layer to the interface between the window layer and the second light-absorbing layer by diffusing a first impurity of a first conductivity type into a portion of the window layer; and forming a main electrode of a first conductivity type on a portion of the diffused region whilst simultaneously forming a main electrode of a second conductivity type on the remaining portion of the window layer.
If this semiconductor photodetector (or photo detecting element) is used for two-wavelength multiplex optical communications, for example, then the energy gap wavelength of the preliminary layer is set such that it is shorter than the energy gap wavelength of the second light-absorbing layer. Furthermore, the energy gap wavelength of the preliminary layer is set such that it is longer than the respective energy gap wavelengths of the substrate, buffer layer and window layer.
Adjustment of the energy gap wavelengths can be achieved by altering the composition of the semiconductor materials forming the layers, for example. Moreover, the energy gap wavelength of the preliminary layer is longer than the wavelength of the light on the shorter wavelength side in the incident light, and it is shorter than the wavelength of the light on the longer wavelength side. The energy gap wavelength of the second light-absorbing layer is longer than the wavelength of the light on the longer wavelength side in the incident light. Thereby, it is possible to manufacture a photodetector (or photo detecting element) whereby, in two-wavelength multiplex optical communications, the shorter wavelength light is absorbed by the first light-absorbing layer, whilst the longer wavelength light is absorbed by the second light-absorbing layer, and an electrical signal originating in the light on the longer wavelength side only can be extracted.
According to preferred embodiments of this manufacturing method, the heating of the substrate carried out for the purpose of crystal growth of the buffer layer, second light-absorbing layer and window layer, after the preliminary layer for the first light-absorbing layer of a second conductivity type has been formed on the first principal surface of the substrate, is used to diffuse a second impurity of a first conductivity type from the substrate into the preliminary layer adjacent to the substrate, thereby forming the region of the preliminary layer adjacent to the substrate into a region of a first conductivity type. The region of the preliminary layer into which the second impurity does not diffuse forms a region of a second conductivity type. Thereby, a first light-absorbing layer comprising a region of a first conductivity type and a region of a second conductivity type is formed. Moreover, a pn junction region is formed within the first light-absorbing layer by this region of a second conductivity type and the region of a first conductivity type. The carriers generated by the light on the shorter wavelength side absorbed by the first light-absorbing layer can be trapped in this pn junction region. Therefore, it is possible to prevent carriers originating in the light on the shorter wavelength side from creating a detrimental effect on the electrical signal originating in the light on the longer wavelength side, which is ultimately to be extracted.
Moreover, according to this manufacturing method, since it is not necessary to provide a short-wavelength shielding filter composed of a material which is different to the materials constituting the photodetector, as used conventionally to shut out light on the shorter wavelength side, there is no risk of deterioration in the structural or operational reliability of the element. Furthermore, no special facilities for installing this filter are required, and hence further complication of the manufacturing process is avoided and costs relating to the manufacture of the photodetector can be reduced.
According to a fourth aspect of this invention, a semiconductor module comprising the aforementioned semiconductor photodetector (or photo detecting element) is provided. This photodetector module comprises, for example: an optical fiber; a ferrule for holding this optical fiber in the center portion thereof; a lens for focusing light emitted from an optical fiber; a semiconductor photodetector onto which the focused light is irradiated: and a header for fixing the photodetector. A semiconductor photodetector according to this invention is used as the semiconductor photodetector in this module. If this module is used as a module for two-wavelength multiplex optical communications, for example, then it is possible to extract an electrical signal originating in the light on the longer wavelength side readily, without needing to provide a short-wavelength shielding filter between the lens and the photodetector. Moreover, since it is not necessary to provide a filter, there is no risk that light reflected at the filter will return to the transmitting side as back-reflected light. Therefore, it is possible to prevent degradation of the reliability of the optical communications system. Moreover, since the light on the longer wavelength side, which is to be extracted, is not attenuated by passing through a filter, it is possible to prevent signal degradation caused by the filter.